Ball transfer units (BTUs) are known for facilitating movement of pallets or cargo containers from one location to another. Generally, each ball transfer unit has a ball or roller, which projects above a "ball deck" and which is capable of freely rolling. The ball contacts a portion of the bottom surface of the container being conveyed along the deck. The pallet or container is conveyed by a person walking along the ball deck and pushing the pallet or container. The pallet or container is moveable along the ball deck by the reduced friction provided by the BTUs in the ball deck.
Prior art BTUs, when used with aluminum air cargo containers, have several operating disadvantages. The prior art BTUs are unable to readily accommodate uneven bottom surfaces of air cargo containers, and have been susceptible to contamination with dirt and other materials, causing increased friction and decreased performance.
Another disadvantage of known BTUs is that the balls have relatively small diameters. Consequently, the balls present relatively small contact areas to support a pallet or container, creating very high loads on the areas of the pallet or container supported by the BTU. This excessive contact stress can damage the containers, which are typically light-weight aluminum, and hence are somewhat fragile. This excessive contact stress requires the pallets or containers to have structural strength in excess of that otherwise necessary to support the loads carried therein, and also requires that there be a substantial number of such BTUs to spread the contact stress over a larger area.
A ball transfer unit, as used in a different field of art, i.e., conveying slabs of hot milled steel, is disclosed in Belgium Pat. No. 510,684. The BTU disclosed in the Belgium patent has two roller balls of the same diameter. The unit is useful primarily with slabs or other materials having a uniform bottom surface. The ball transfer unit is unable to accommodate uneven bottom surfaces of containers or pallets, for example, aluminum air cargo containers, which typically have uneven bottom surfaces to enhance their structural integrity. Furthermore, the design is susceptible to contamination of the ball bearings with dirt and other contaminants, which may enter through the top of the housing. Thus the contaminants are capable of interfering with the operation of the ball bearings, sometimes resulting in BTU failure.
What is desired, therefore, is a ball transfer unit which minimizes the force necessary to move a container, which is capable of accommodating uneven bottom surfaces of the container, which reduces the ability of dirt and other contaminates to interfere with the operation of the unit. Further, such a desirable ball transfer unit should permit the use of lighter weight cargo containers, which is of critical importance when the container is used in connection with air cargo, where every saved ounce of weight will save the airline the cost of the fuel needed to lift that weight.